1. Field of the Invention
The present invention relates generally to an apparatus and method for a cell search and demodulation in an asynchronous mobile communication system, and in particular, to an apparatus and method for generating reference timing for a cell search and demodulation.
2. Description of the Related Art
In general, a mobile communication system can be classified into a synchronous system that is used in the United States and an asynchronous system that is used in Europe for 3rd generation mobile communication.
With the rapid development of the mobile communication industry, a future mobile communication system, which provides not only the general voice service but also data and image services, has been developed, and standardization works thereon are being carried out. However, as stated above, the United States and Europe use different standardizations for mobile communication systems. A European future mobile communication system is called a 3GPP W-CDMA (3rd Generation Partnership Project Wideband Code Division Multiple Access) mobile communication system. In the W-CDMA mobile communication system, Node Bs are not synchronized with one another. Thus, in order to identify the Node Bs, unique scrambling codes are assigned to the Node Bs. For example, if the number of cells, or Node Bs constituting the W-CDMA mobile communication system is 512, each of the 512 Node Bs is assigned its own unique scrambling code among 512 scrambling codes.
Meanwhile, in the W-CDMA mobile communication system, a UE (User Equipment) must recognize a scrambling code assigned to a serving Node B from which the UE desires to receive a service. Therefore, the UE must perform an operation of determining a scrambling code of a signal received at a highest power among the signals received from neighboring Node Bs. This operation is generally called a “cell search.”
In the W-CDMA mobile communication system which assigns scrambling codes as stated above, a UE uses a general cell search algorithm for examining phases of all the assignable scrambling codes, for a cell search. However, such a general cell search algorithm is inefficient, since it requires a long cell search time.
To solve this problem, a multistep cell search algorithm has been proposed. In order to realize the multistep cell search algorithm, 512 scrambling codes are divided into 64 code groups, and each of the 64 code groups is assigned 8 scrambling codes. Further, in order to easily perform the cell search, a synchronization channel (SCH) signal and a common pilot channel (CPICH) signal are used. The synchronization channel includes a primary synchronization channel (P-SCH) signal and a secondary synchronization channel (S-SCH) signal. The SCH signal and the CPICH signal are provided from a Node B to a UE over a downlink.
The multistep cell search algorithm includes first to third cell, search steps. In the first cell search step, a UE synchronizes a slot time of a slot received at maximum power, using a P-SCH transmitted from a Node B. In the second cell search step, the UE performs frame synchronization and detects a Node B group designation code for a Node B to which the UE belongs, through an S-SCH transmitted from the Node B in the slot time synchronized state. In the third cell search step, the UE finally searches a Node B where the UE itself belongs by detecting a scrambling code of a Node B, using a CPICH transmitted from the Node B, based on the frame synchronization and the Node B group designation code searched in the second cell search step.
FIG. 1 illustrates a structure of an SCH and a CPICH used for cell search in a general W-CDMA mobile communication system. Referring to FIG. 1, one frame is comprised of 15 slots. A P-SCH and an S-SCH are transmitted by N=256 chips at the beginning of each slot, and the two channel signals are overlapped since orthogonality is maintained between the channel signals. For the CPICH signal, each Node B uses its own unique scrambling code, and a period of the scrambling code is equal to a length of one frame. In a W-CDMA mobile communication system having this channel structure, only one-frame length of a Gold code with a period 218-1 is used as the scrambling code, and only M=512 Gold codes among the available Gold codes are used.
A primary synchronization code cp used for the P-SCH is commonly used for all cells (or Node Bs), and only a 256-chip period, 1/10 of one-slot period, is repeatedly transmitted at the beginning of each slot. The P-SCH is used by a UE to search slot timing of a received signal. That is, the UE receives the P-SCH and synchronizes a Node B slot time by a primary synchronization code cp included in the received P-SCH (First Cell Search Step).
The S-SCH is mapped with secondary synchronization codes for Node Bs, i.e., Node B group designation codes csi,1˜csi,15 during transmission, and a Node B time-slot-synchronized by the P-SCH detects a Node B group designation code and frame synchronization through the S-SCH. Here, the Node B group designation code is information for determining a cell group where a Node B belongs, and a comma free code is typically used for the Node B group designation code. The comma free code is comprised of 64 codewords, each codeword is comprised of 15 symbols, and the 15 symbols are repeatedly transmitted each frame. However, the 15 symbols are mapped to one of the secondary synchronization codes csi,1˜csi,15 during transmission, as stated above. That is, as illustrated in FIG. 1, an ith secondary synchronization code corresponding to a symbol value i is transmitted each slot. The 64 codewords of the comma free code distinguish 64 code groups. The comma free code is characterized in that each codeword has a unique cyclic shift. Therefore, it is possible to acquire information on a code group and frame synchronization by correlating the secondary synchronization codes with the secondary synchronization channel signal for several slot periods and then checking 64 codewords and 15 cyclic shifts of each codeword. Here, the term “frame synchronization” means synchronization for timing or phase within one period of a scrambling or spreading code in a spread spectrum communication system. In the latest W-CDMA mobile communication system, since one period of the spreading code and a length of the frame are both 10 ms, this will be called frame synchronization (Second Cell Search Step).
By performing the second and third cell search steps, the UE acquires information on slot synchronization, Node B group designation code, and frame synchronization through the P-SCH and the S-SCH. However, the UE does not recognize yet a scrambling code for a Node B where the UE itself belongs, among 8 scrambling codes within a code group based on the acquired Node B group designation code, so code synchronization has not been completed yet.
Therefore, the UE can identify a scrambling code to use among the 8 scrambling codes by taking correlation between the 8 scrambling codes belonging to the code group for the CPICH (Third Cell Search Step).
Meanwhile, the UE must periodically check power levels of signals from the serving Node B and its neighboring Node Bs in order to receive an optimal Node B multipath signal in a radio channel environment or a handoff state. In this case, the UE acquires timing information of the neighboring Node Bs either by the multistep cell search algorithm or from the Node B, and then periodically takes correlation on the CPICH from a corresponding Node B. In the following description, unlike the cell search performed when timing of neighboring Node Bs is not acquired, the process of periodically checking power levels of signals from the serving Node B and its neighboring Node Bs will be called a “multipath search.”
Generally, a Node B distinguishes signals from the Node Bs that will probably move, by cell search, and continuously manages the distinguished Node B signals through the multipath search. If received signals are determined as valid multipath signals by the cell search and the multipath search, the UE demodulates the corresponding multipath signals.
As stated above, the W-CDMA mobile communication system acquires code synchronization while continuously performing the 3-step cell search. That is, in the first step, synchronization on time slot is acquired using the P-SCH, and in the second step, frame synchronization and a scrambling code group are acquired using the S-SCH. Finally, in the third step, the UE searches the CPICH after frame synchronization, and finds out a scrambling code assigned to a corresponding Node B among the 8 available scrambling codes within the code group.
In order to perform such a cell search process, it is necessary to determine a reference time when the first step will be started, and a time when the second step will be started after slot synchronization is acquired in the first step. In addition, for code group discrimination and frame synchronization acquisition in the second step, it is necessary to recognize the start point in order to determine a frame boundary from the number of cyclic shifts performed during codeword decoding. Finally, it is necessary to determine a time when the third step will be started.
Meanwhile, since the cell search process must be performed not once but continuously, the UE needs a certain criterion in order to calculate and manage a timing difference between a frame boundary of a currently modulated signal and a frame boundary of signals, to be probably demodulated, from the same Node B or different Node Bs. That is, the UE should be able to calculate a difference (offset) between a frame boundary of a neighboring Node B, detected by the cell search, and a frame boundary of another Node B, detected by the previous cell search.
In addition, the multipath search detects power levels of received signals by performing a correlation process on scrambling codes for the Node Bs that will probably move. However, since frame boundaries of the Node Bs are different from one another, the scrambling codes must be initialized before the correlation, such that the frame boundaries are aligned with a code phase of each Node B. Therefore, it is necessary to define reference timing for initializing scrambling codes for the Node Bs.
A multipath signal component determined as a valid signal by the cell search and the multipath search is assigned to a demodulator (or finger) in the UE, for demodulation. Therefore, in the demodulator of the UE, a scrambling code for the corresponding Node B must be initialized in alignment with a frame boundary of the corresponding multipath signal, and reference timing indicating this time point must be defined.